Let the sun shine in

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The drive for clean technology has resulted in a ‘race to the top’ for the photovoltaic research community, says Prof Ravi Silva

It is nearly 40 years since the discovery of electrically conducting polymers by Prof Alan Heeger, Nobel Laureate, and nearly 30 years since the first organic heterojunction photovoltaic (PV) device was reported. Given the concentration of research undertaken around the world, organic (including organic-inorganic hybrid) solar cell technology is now a maturing technology, with academic breakthroughs progressing in increasingly rapid timescales.

The drive for cost-effective, clean technology has resulted in a ‘race to the top’ for the PV research community. Innumerable headlines point to ‘hero’ devices that deliver ‘miracle’ discoveries; however, there are issues in giving these reports the ‘benefit of the doubt’.

The reporting of organic PV (OPV) device and material performance mirrors that of a number of technologies in recent years, where discovery is hyped and, after initial interest, becomes undervalued by commercial representatives.

This issue has undermined the solar cell research community. Based on a study by Zimmerman et al, carried out on 375 publications in peer-reviewed journals, the authors identified (by comparing the short-circuit current density obtained through the current-voltage sweep versus quantum efficiency measurements) that a significant fraction (more than 37 per cent) of publications overestimated device performance.

Robust reporting methodologies and measurement protocols are not being widely adopted in the PV research community. This may be because in the initial stages of PV development there was no clear understanding of the protocols. However, it is now nearly a decade since Shrotriya et al described measurement procedures for characterising organic PV.

Some experts believe that perovskites will in future challenge crystalline silicon in PV devices

With the ever-increasing focus on obtaining higher device power conversion efficiencies (PCEs) for OPV, there is a need to ensure devices are measured accurately.

Typically, the devices reported within this ‘race to the top’ are single devices with little evidence of repeatability or simple statistical analysis of a sample population. This gives an overestimate of performance capabilities, undermining the technology in the eyes of industry.

National measurement institutes offer standardised measurement facilities for ascertaining PV device performance, but are invariably expensive and time consuming. Given that a researcher may change the formulation of OPV layers on a weekly basis, the process is not cost effective for researchers. This causes research groups to report against similar reference devices fabricated in their laboratories, with the associated variation in production and measurement quality.

Venturing beyond selective estimates of performance, it is also sobering to note how these reports are erroneously placed in the context of the research field as a whole. Consider the well-known PV efficiency chart created by the National Renewable Energy Laboratory (NREL) as a guideline. Although researchers place confidence in the chart, it needs to be questioned if the chart itself is the best source for the comparison of technologies.

For example, can perovskites with 18 per cent efficiency over a few square millimetres be compared to a crystalline silicon cell that can deliver 25 per cent over ~144cm2?

Moreover, the former have been indicated on the NREL chart as being unstabilised. So is the comparison often made with silicon PV technology appropriate?

Shouldn’t a system that is considered to be a promising contender as a future PV technology display a sufficient level of stability for it to be incorporated into such an important performance chart? This is not to devalue the potential of perovskite technology, once scaling and manufacturability of large-area devices has been achieved. But in the case of crystalline silicon, the technology took more than two decades to mature, and expecting solution-processed PV products to be purchased over the counter in a few years enters the realm of fantasy.

Clearly, the research community should be careful, as any incorrect belief formed in the development of the technology will, over time, lead to a lack of confidence in the field. Similarly, policy makers and funders need to note the time spans required to deliver real-world products.

Although the ultimate aim is to contribute to finding a solution to the renewable energy problem, it is imperative that performances reported for PV technologies become aligned with ‘best practice’ so we build on the work carried out in laboratories around the world towards a greener and more sustainable future.

Solar cell technology has been a focus of Surrey University’s Advanced Technology Institute (ATI) since 2002. Its work has focused on the application of inorganic nanotubes and nanoparticles in OPV devices. Working directly with industry has resulted in the integration of carbon nanotubes that act as additional charge generation sites that help increase the current extracted and improve device performance. Industrial collaboration has also ensured that the ATI focuses on devices whose performance values are realistically achievable during the transfer of knowledge from lab to manufacturing plants.

This article is based on correspondence from the Advanced Technology Institute in the journal Nature Photonics, published April 9 2015.

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As I understand it about 1kW per sq.m. of sunlight energy is the maximum obtainable anywhere on earth so this research has a natural ceiling of achievement. To achieve more would require rewriting the inverse square law.

At some point the law of diminishing returns must kick in and I believe we may be close to that now. Further ‘advanced’ research is thus either a pure science exercise with little economic benefit or just possibly a cynical grab for grant money.

I live in the east of England where rainfall is low and available solar energy is possibly higher than most other areas of the UK. It is also moderately flat. This presumably is the reason Solar ‘farms’ are appearing in ever greater numbers around here, removing valuable farmland from food production. Has anyone done the sums equating the need to increase food imports (food mile energy cost)against the Electrical energy yield of such installations. They are also remarkably ugly, on a par with current designs of wind powered electrical generation.

Why would ‘they’ need to: that will come under some-one-else’s budget and as the intellectual pigmys and clerks who decide these things for us simple Engineers have ably shown, that makes such losses disappear. I am reminded of the story of the economics to blow a hole in a Viet Cong supply track. This cost the USAF $2,000 -it cost the VC $2.00 to repair it.

Solar Farms are a waste of space as long as there are parking lots in abundance that are open to the sun. Cover all these parking lots with a roof and put PV on those roofs.
For supermarkets this has the added benefits for the customer that in summer their car will heat up less and they’ll need less energy for airconditioning. And when it’s raining they will get dry to their car.

And there are numerous other roofs that could be covered in PV. Those roofs just have to be tested for stability.

Good suggestion, Ralf. There are thousands of acres of car parks and warehouse and other roofs that could be “solarised”. If all new houses were required to have solar roofs the costs would tumble.

Regarding maximum solar intensity, by using heliostat mirrors the solar intensity can be concentrated many times which is the basis for concentrated solar thermal power stations proposed for the world’s desert regions. By means of HVDC cables this power coulld be transmitted to distant users with only 3% loss per 1000 kms. See “Sustainable Energy – without the hot air “at http://www.withouthotair.com

As M. Brooks says, solar energy can be concentrated, but the 1kW/sq.m. ‘rule of thumb’ still applies to the overall catchment area. To install solar farms in high sunshine zones means a high cost for safely installing HVDC lines covering 1000’s of kilometres. As Mike Blamey says. The bean counters generally take a very narrow (& some would say, cynical) view of cost/benefit calculations.
Jan is right about the weight of solar panels in respect of roofing over large car parks. Note also the significant loss of useable parking space from the need for roof supports, which means larger car parks, and so on. I would also factor in the longevity of solar panels and the cost for regular cleaning to maintain anywhere near original efficiency levels. To me solar is fine for home and factory roofs but not for commercial ‘farms’. My opinion is they are another Grant grabber just like Wind Powered Electrical generation.

Some years ago, I came across a statistic I had no reason to question. Our space-ship ‘Earth’ receives one /two-billioneth (that’s 1 over 2,000,000,000) of the energy that our friendly nearest star offers to its ‘system’ every second. Not a lot! The rest passes us by! -as we only are able to be ‘impinged’ upon (let alone use) by the tiny proportion that ‘hits’ us. The solution (if we need one) is surely simple. Have those ‘sails’ -the solar panels we have learnt to attach to space craft as a part of our ‘collection system’. Mankind’s harnessing of wind-power was the vehicle for most travel and trade for at least two thousand years: we are surely able to do better than we do at present!

Here another novelty: use the light of the sun instead of artificial light.

In Hong Kong, most high rise buildings close their windows and curtains, and then use artificial light because it’s too dark inside.

Just do the math in your own office how many kW are used just for lighting.

Using daylight saves a lot of energy. Use horizontal blinds instead of vertical blinds. Use the blinds to reflect incoming light to the ceiling and from there further into the room. Or use light tunnels from the roof into some dark corner, e.g. stairs, under stairs, corridors, etc.

Absorption refrigeration systems work with heat instead of a rotating compressor. Concentrate sunlight and you have heat that can drive the cooling system. Make the refrigerator as large as a building and you have sun-powered airconditioning that works exactly when you need it – lots of sun.

Add some PV to drive fans and recycle the heat from used air. Lots of comfort for little external (bought) energy input.

Sun and wind and tides are coming in free. Using them also solves the problem of transporting energy from unstable political entities. And with less need to protect those trade routes, there’s less wastage for defense expenditures.
Use that money to build libraries and outdoor swimming pools (to avoid kids drowning in reservoirs).

Mike – In the same vein, the ultimate surely would be the ‘Ringworld’ as developed by, but not invented by, the SF author Larry Niven in various tales of known space…

Orbiting solar collectors are a fine idea but how to get the energy back to Earth in concentrated form, as I believe it would have to be, without risking the demolition of huge areas if the delivery system fails, may be the main issue with this, notwithstanding any cost/benefit equations.

The comment about solar farms or car parks prompted an idea in me. In southern Italy several car parks have covered bays to protect the cars from the scorching sun. Why not cover the car park spaces with solar cells and park the cars underneath – two uses for the space of one! We could also charge electric cars directly while they’re parked.